I am quite interested in getting a Nokia N8 when my contract comes to an end soon. However I do have some reservations as I have seen customers complaining about it's battery life. Irrespective of that, a pay as you go N8 on the O2 network is only £300! That price comes with a price- it is locked to the O2 network and at the moment, it is still not possible to unlock on the O2 network yet. If you want to unlock it, you would have to wait for a year before O2 will provide you with the unlocking code or wait for an unlocking code to be made available later.
With a price tag of £300, it is the cheapest of all! Nokia is selling the phone at around £429 but you could get it cheaper when you do a search on ebay.